The speed of rotation of an induction machine deviates from the frequency of the supplying network in a known manner by a certain slip. When operated as a motor, the speed of rotation is somewhat lower than the supplying frequency divided by the number of pole pairs, i.e. the synchronous speed in motor operation and, correspondingly, the speed of rotation in generator operation is somewhat higher than the network frequency divided by the number of pole pairs. The magnitude of the slip depends on the load of the machine, which is why adjusting the speed of rotation often leads to a complicated control system, because the variable quantities cannot always be measured accurately. For this reason, it is difficult to maintain a constant speed of rotation of an induction machine, especially when the load varies.
There are several previously known control systems for adjusting the speed of rotation of an asynchronous motor to a level corresponding to a reference value. These solutions have often led to the use of complicated and expensive regulating apparatuses which could seldom be realized in the case of small motors. Besides, the proposed apparatuses have required the measurement of the speed of rotation directly from the motor shaft or the measurement of the load current of the motor.